Chiral Organic Structure-Directing Agents

Abstract

Chirality is crucial for life. The preparation of enantiopure chiral compounds is highly desirable in the chemical industry, especially in the pharmaceutical sector. In this context, the design of chiral solids able to discriminate between enantiomers of chiral compounds, either during adsorption or asymmetric catalytic processes, is one of the greatest challenges nowadays in chemical research. Zeolite-type materials represent ideal candidates to achieve enantioselective chiral solids since they could combine their high stability, surface area, and shape-selectivity with a potential enantioselectivity that could be enhanced by the confinement effect. Despite the occurrence of chiral zeolite frameworks and the strong interest in preparing these chiral solids, very little success has been met in preparing these in homochiral form. The main strategy to induce chirality in zeolite materials has been the use of chiral structure-directing agents, in an attempt to transfer their chiral feature into the nascent zeolite structure. However, although many chiral organic species have directed the crystallization of zeolite frameworks, some of them even being chiral, there is only one unique very recent example of success in transferring the chirality from the organic structure-directing agent into an enantioenriched chiral zeolite material. Chiral coordination compounds have been very successful in transferring their chirality onto inorganic frameworks through the development of extensive H-bond host–guest interactions, but these chiral materials usually collapse upon removal of the guest species. In this chapter we report the different types of chiral molecules, both organic and organometallic compounds, used so far as structure-directing agents in an attempt to promote the crystallization of homochiral zeolites; we analyze in detail the possible reasons for the general failure in transferring their chirality, and we propose approaches to prepare known chiral zeolite frameworks in homochiral form. Furthermore, we also review a different approach we have followed in our group in order to induce chirality in zeolite materials, consisting in the development of chiral spatial distributions of dopants embedded in otherwise achiral zeolite frameworks.